What is required in the ninth commandment?

Heidelberg Catechism (extended)

The Catechism
Method of Instruction in the Christian Religion
As the Same is Taught in the Reformed Churches and Schools

(with the Scripture references written out)

Note. This Catechism is fully based on the Scriptures. The references to Scripture are indicated in parentheses with a letter. For example, the letter (a) points to the texts (a) placed after the answer.

43. Lord's Day 

Q. 112. What is required in the ninth commandment? 

A. That I bear false witness against no man, (a) nor falsify any man's words; (b) that I be no backbiter, nor slanderer; (c) that I do not judge, nor join in condemning any man rashly, or unheard; (d) but that I avoid all sorts of lies and deceit, as the proper works of the devil, (e) unless I would bring down upon me the heavy wrath of God; (f) likewise, that in judgment and all other dealings I love the truth, speak it uprightly and confess it; (g) also that I defend and promote, as much as I am able, the horror and good character of my neighbour. (h) 

(a) Prov.19:5 A false witness shall not be unpunished, and he that speaketh lies shall not escape. Prov.19:9 A false witness shall not be unpunished, and he that speaketh lies shall perish. Prov.21:28 A false witness shall perish: but the man that heareth speaketh constantly. 

(b) Ps.15:3 He that backbiteth not with his tongue, nor doeth evil to his neighbour, nor taketh up a reproach against his neighbour. Ps.50:19 Thou givest thy mouth to evil, and thy tongue frameth deceit. Ps.50:20 Thou sittest and speakest against thy brother; thou slanderest thine own mother's son. 

(c) Rom.1:29 Being filled with all unrighteousness, fornication, wickedness, covetousness, maliciousness; full of envy, murder, debate, deceit, malignity; whisperers, Rom.1:30 Backbiters, haters of God, despiteful, proud, boasters, inventors of evil things, disobedient to parents, 

(d) Matt.7:1 Judge not, that ye be not judged. Matt.7:2 For with what judgment ye judge, ye shall be judged: and with what measure ye mete, it shall be measured to you again. Luke 6:37 Judge not, and ye shall not be judged: condemn not, and ye shall not be condemned: forgive, and ye shall be forgiven: 

(e) John 8:44 Ye are of your father the devil, and the lusts of your father ye will do. He was a murderer from the beginning, and abode not in the truth, because there is no truth in him. When he speaketh a lie, he speaketh of his own: for he is a liar, and the father of it. 

(f) Prov.12:22 Lying lips are abomination to the LORD: but they that deal truly are his delight. Prov.13:5 A righteous man hateth lying: but a wicked man is loathsome, and cometh to shame. 

(g) 1 Cor.13:6 Rejoiceth not in iniquity, but rejoiceth in the truth; Eph.4:25 Wherefore putting away lying, speak every man truth with his neighbour: for we are members one of another. 

(h) 1 Pet.4:8 And above all things have fervent charity among yourselves: for charity shall cover the multitude of sins.

Source: http://www.ccel.org/creeds/heidelberg-cat-ext.txt